In the state of the art, numerous membranes are known that are made from tubular filter elements. Thus, a filter element of the multichannel type is known that includes a rigid porous support of elongate shape having a right cross-section that is polygonal or circular. The porous support, which is made of ceramics for example, is organized to include a series of channels that are parallel to one another and to the longitudinal axis of the porous support, each having a right cross-section that is circular. The surfaces of the channels are covered in at least one separator layer of nature and morphology adapted to separate molecules or particles contained in the liquid medium flowing inside the channels. By the sieve effect, such a membrane separates out the molecular or particulate species of the substance to be treated insofar as all molecules or particles of diameter greater than the diameter of the pores in the membrane are stopped. During separation, fluid transfer takes place through the separator layer(s), after which the fluid spreads out within the pores of the support and travels towards the outside surface of the porous support.
A major drawback of such multichannel filter elements lies in the resulting low flow rate of filtrate. The path which the filtrate must follow before reaching the outside surface of the support, when starting from the channels situated in the central portion of the support, is much longer than the path that needs to be followed from the other channels, in particular from the peripheral channels. Also, filtrates coming from channels in the central region of the support encounter filtrates coming from other channels. That is why headloss appears in the transfer of filtrate towards the outside surface of the support. This headloss opposes the transfer pressure and reduces the flow speed.
In an attempt to remedy the above drawback, patent application WO 93/07959 proposes a tubular filter element each of whose channels is of non-circular right cross-section. In a first embodiment, the filter element comprises an inorganic porous support in which the channels are formed parallel to the central axis of the support, being disposed substantially in a circle that is coaxial with the central axis. In right cross-section, each channel has a peripheral wall directed towards the outside surface of the support and co-operating therewith to define a passage of constant thickness through which filtrate is conveyed. Each peripheral wall is extended at both ends by radial walls that are connected together, and each of which co-operates with the facing radial wall of an adjacent channel to form a partition. The profile of the channels is selected in such a manner as to leave wedge-shaped partitions that flare towards the outside of the substrate. In a second embodiment, the axes of the channels are situated either on a plurality of circles that are coaxial about the axis of the support, or else in a plurality of layers that are parallel to one another and to the axis of the support. In that second embodiment, adjacent channels between two layers leave a partition that flares likewise towards the outside of the substrate. It can thus be seen that the wedge shape of the partitions defined between the radial walls facilitates transfer of permeate towards the outside surface of the support.
It also appears that by making channels that are not circular in section, such an element makes it possible to increase the ratio of filter area over the volume of porous support used. Nevertheless, it must be observed that such an element suffers from a major difficultly of implementation. In order to better occupy the section of the porous support so as to increase filter area, the above-specified patent application describes, in FIG. 3 thereof, a variant embodiment in which the channels can be classified in three different categories. The channels of the first category have their axes situated on a circle that is coaxial about the central axis of the support. All of those channels present both a right cross-section that is identical and can be referred to as pentagonal, and a hydraulic diameter that is identical. Given the non-circular shape of the channels, the diameter of each channel is defined by its hydraulic diameter .phi.h such that .phi.h=4. S/P where S is the section of the channel and P is the perimeter of the channel. The channels of the second category have their axes situated on a circle that is coaxial about the central axis of the support but that is situated inside the circle on which there are situated the axes of the channels of the first category. All of the channels of the second category have both identical right cross-section that can be referred to as triangular, and also identical hydraulic diameter. A third category comprises a channel of circular section centered on the central axis of the porous support.
An examination of that type of element has led to the observation that the right cross-sections of the channels are different for the three categories of channel. As a result, the speed of fluid flow in the channels of the three categories is different since said speed is equal to the flow rate along the channel divided by the right cross-sectional area of the channel. Flow speed is one of the parameters contributing to the value of shear stress, which stress is created at the wall of the element because the element operates under cross-flow conditions. In theory, such operation favors unclogging of the surface of the filter element, due to the shear stress that removes deposited matter. In the filter element described above, cross-flow unclogging is not uniform in all of the channels because of their different flow speeds, thereby leading to misoperation of the filter element.
Also, the desire to optimize the ratio of filter area over the volume of the porous support used leads to making a filter element of the type described above in which the channels have hydraulic diameters of different values. In certain applications, in which the fluid to be treated is of a heterogeneous nature, it can be observed that channels of too small a hydraulic diameter become blocked.
Although making a filter element having channels of non-circular section presents an undeniable advantage with respect to increase in filter area over volume of the porous support used, it must be observed that there remains a problem in defining the dimensions and the shape of the channels in order to avoid non-uniform unclogging between the channels and blocking of some of the channels by the fluid to be treated.
There is thus a need to define characteristics for a filter element so that it is designed to optimize the ratio of the filter area over the volume of the porous support used while enabling uniform unclogging to be obtained in all of the channels and no obstruction of the channels by the fluid to be treated.